A-bomb survivors, Atomic Energy Commission--Department of Energy workers, and the population exposed to fallout from atmospheric tests at the Nevada Test Site have been studied for their potential to provide low-dose risk estimates for radiogenic cancer. Only some combination of epidemiologic studies at higher doses, with experimental and theoretical work, will provide a reliable guide to such risks. Sources of variation in risk estimates for radiogenic cancer are explored for their significance to research on carcinogenic mechanisms and to give direction to epidemiologic research.